Impact-operated boring tools are well-known in the art. Sudnishnikov et al. U.S. Pat. No. 3,756,328 discloses one such device. Impact-operated boring tools are used for burrowing holes in soil, particularly horizontal or near horizontal passages for installation of utility lines when trenching is undesirable. As the name implies, such boring tools function by impact. The tools possess a striking member (striker) slidable within a cylindrical housing. The striker delivers impacts on a surface at the front end of the housing. This impacting motion within the tool itself causes the soil around the tool to compact away from the nose of the housing, thus forming a hole.
The movement of the striker against the front surface is accomplished through the supply of pressurized fluid (such as compressed air) to a chamber behind the striker. Reciprocal movement is accomplished through the use of a control sleeve and ports in the striker. When the striker reaches a particular point in its forward path, the ports move past the sleeve to define an opening between the chamber behind the striker and the chamber in front of the striker. This allows the compressed air to pass to the chamber along the sides and in front of the striker. Because the cross-sectional area of the chamber in front of the striker is larger than the chamber behind the striker, the compressed air in the front chamber then forces the striker backwards. As the striker moves backwards, the opening defined by the ports is closed. When the striker reaches a particular point in its rearward path, the ports in the striker again move past the control sleeve to define an opening between the front chamber and exhaust passages leading to the atmosphere. The compressed air from the front of the striker is thus exhausted to the atmosphere. At this point, the pressure inside the chamber behind the striker again becomes greater than the pressure in front of the striker. Consequently, the striker begins to move forward once more.
Reversible impact-operated boring tools are also well-known in the art. Kostylev et al. U.S. Pat. No. 4,683,960 discloses such a device. A reversing mechanism is often necessary to retrieve the tool from the hole being burrowed in case the tool encounters an obstruction in the soil or deviates greatly from a straight path.
Over the years, numerous attempts have been made to improve the safety and reliability of the reversing mechanisms. Trying to simplify the means for switching from the forward to the reverse mode of operation often resulted in uncertainty about which direction the machine was traveling in the hole. It seemed that the simpler it was to switch modes, the easier it was to switch accidently. Apart from the obvious danger this posed to the operators of the tool, this could also be very time consuming. If an operator were to switch modes accidently, time thought to be spent on burrowing may actually be time spent on retrieving the tool unwittingly. The error would not be discovered immediately, thereby wasting valuable operation time.
The prior art discloses various means for accomplishing reverse motion. Some require interrupting the pressurized fluid supply. Others require manipulation of the hose supplying the pressurized fluid to the tool, either by rotating the hose or by pulling it back. Still others require both the interruption of the pressurized fluid supply and the manipulation of the hose. However, each means has its disadvantages.
Edward J. Bouplon U.S. Pat. No. 4,662,457 discloses a reversing mechanism requiring both means. The pressurized fluid supply must be terminated and then the hose must be rotated approximately one quarter turn clockwise in order to switch to the reverse mode of operation. Sometimes, when the pressurized fluid supply is terminated and the tool is therefore shut off, the tool does not restart when the pressurized fluid supply is recommenced. Helmuth Roemer U.S. Pat. No. 4,840,237 discloses a reverse mechanism requiring that the hose be rotated. When the hose is flexible, it is often difficult to relate the degree of rotational motion of the hose at the surface to the degree of rotational motion at the tool itself, which may be some distance away. Consequently, it is often difficult to reverse the operation of the tool, or to be certain of the direction of operation.
Kostylev et al. U.S. Pat. No. 4,683,960 discloses a reversing mechanism that requires applying sufficient force to a steel cable surrounding the air supply hose to overcome the compression force of a spring within the cable. Compression of the spring enables reverse operation of the tool. An alternate embodiment of the invention depicts a flanged tube within the air supply hose for accomplishing the same result as the steel cable--compression of the spring. There is no way of knowing whether the tension force is sufficient to overcome the compression force of the spring, which may be some distance away, in order to reverse the direction of operation. Consequently, the uncertainty concerning which direction the tool is operating remains.
Sudnishnikov et al. U.S. Pat. No. 4,214,638 is an earlier patent which discloses a reversing mechanism that does not require manipulation of the fluid supply hose. The invention employs a control valve for alternately supplying compressed air or suction to the boring tool. When suction is applied, a control element within the tool is displaced. The tool operates in the reverse mode when compressed air is then resupplied. To switch back to the forward mode, suction is re-applied. This causes the control element to be displaced back to the position for forward movement. While no hose manipulation is required in the above invention, the exact same procedure is employed for switching from forward to reverse mode. Consequently, uncertainty regarding which direction the tool is operating remains.
Paul Schmidt U.S. Pat. No. 4,250,972 issued on Feb. 17, 1981 discloses a patent employing a second compressed air supply. The patent claims to disclose a method for reversing operation of impact-operated boring tools that does not require any hose manipulation and which assures starting of the ram borer in any position along a borehole. Reverse motion is achieved when the second compressed air supply is initiated.
The impacting motion within the tool presents some problems associated with the service-life of the tool. Most tools contain a sleeve made of an elastomeric material within the tailpiece assembly to dampen some of the shocks emitted by the tool in operation. The sleeve is placed between the fluid inlet tubes and the tailpiece, and is usually glued to both. It is the gluing in this region which has presented the problems. The glue must be carefully chosen to be strong enough to withstand the shocking motion. However, the attachment becomes weakened as the glue ages and dirt gathers in the region of the gluing, thus the service-life of the tool is decreased.
Due to the uncertainty presented by the current means for reversing operation of impact-operated boring tools, and the increased labor and time often involved, an alternate means for reversing operation quickly and safely is needed. Due to the decrease in service-life associated with current shock dampening means in tailpiece assemblies, an alternate assembly is needed.